1. Field of the Invention
The invention relates to apparatus for detecting the presence or absence of a mark. More specifically, the invention relates to such an apparatus for detecting the presence or absence of a coloured area at a predetermined position on a surface having a background of a colour different from the coloured area, or of a different shade of the same colour as the coloured area.
2. Description of Prior Art
In many instances, it is necessary that marks be located at specific positions on carriers. For example, in the printing arts, texts or designs must be located at specific positions on cartons.
In the printing of complex folding cartons, there is often a need to go through several types of printing and converting operations to provide cartons which have outer surfaces with predetermined texts and/or designs. As the number of converting steps increases, the possibility of having some of the sheets, which are subsequently folded into the cartons, where one or several of the processes have been omitted increases significantly. In some markets, the absence of some colour, text copy or other elements is not only unacceptable from an aesthetic point of view but can lead to stiff legal penalties. For example, the warning notice on cigarette packages and the UPC symbol on pharmaceutical products.
The prior art has teachings which attempt to solve some of the problems encountered above. For example, U.S. Pat. No. 3,735,097, Zeitlin, May 22, 1973, U.S. Pat. No. 4,383,275, Sasaki et al, May 10, 1983 and U.S. Pat. No. 4,392,056, Weyandt, Jul. 5, 1983. The '097 patent teaches a device for detecting the presence of embossing on background material. The '275 patent normalizes to a white level and this level is used to compensate for the non-uniformity of the sensor outputs. The '056 patent is a detector for detecting control markings of an article such as a moving web.